Messages in this thread | | | Subject | Re: [RFC PATCH v2 1/2] uaccess: Check no rescheduling function is called in unsafe region | From | Valentin Schneider <> | Date | Mon, 14 Jan 2019 12:03:26 +0000 |
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Hi,
On 03/12/2018 13:55, Julien Thierry wrote: > While running a user_access regions, it is not supported to reschedule. > Add an overridable primitive to indicate whether a user_access region is > active and check that this is not the case when calling rescheduling > functions. > > Also, add a comment clarifying the behaviour of user_access regions. > > Signed-off-by: Julien Thierry <julien.thierry@arm.com> > --- > include/linux/kernel.h | 6 ++++-- > include/linux/uaccess.h | 11 +++++++++++ > kernel/sched/core.c | 19 +++++++++++++++++++ > 3 files changed, 34 insertions(+), 2 deletions(-) > > I'm not sure these are the best locations to check this but I was hoping > this patch could start the discussion. > > Should I move the check? Should I add a config option to conditionally > build those checks? >
I was going to say it's already under DEBUG_ATOMIC_SLEEP, but that's only true for the __might_sleep() bit actually.
I think it'd make sense to blanket that under a config, but using DEBUG_ATOMIC_SLEEP for that is a bit too much. What about a DEBUG_UACCESS_SLEEP?
> diff --git a/include/linux/kernel.h b/include/linux/kernel.h > index d6aac75..fe0e984 100644 > --- a/include/linux/kernel.h > +++ b/include/linux/kernel.h > @@ -237,11 +237,13 @@ > struct pt_regs; > struct user; > > +extern void __might_resched(const char *file, int line); > #ifdef CONFIG_PREEMPT_VOLUNTARY > extern int _cond_resched(void); > -# define might_resched() _cond_resched() > +# define might_resched() \ > + do { __might_resched(__FILE__, __LINE__); _cond_resched(); } while (0) > #else > -# define might_resched() do { } while (0) > +# define might_resched() __might_resched(__FILE__, __LINE__)> #endif > > #ifdef CONFIG_DEBUG_ATOMIC_SLEEP > diff --git a/include/linux/uaccess.h b/include/linux/uaccess.h > index efe79c1..50adb84 100644 > --- a/include/linux/uaccess.h > +++ b/include/linux/uaccess.h > @@ -266,6 +266,13 @@ static inline unsigned long __copy_from_user_inatomic_nocache(void *to, > #define probe_kernel_address(addr, retval) \ > probe_kernel_read(&retval, addr, sizeof(retval)) > > +/* > + * user_access_begin() and user_access_end() define a region where > + * unsafe user accessors can be used. > + * During execution of this region, no sleeping functions should be called. > + * Exceptions and interrupt shall exit the user_access region and re-enter it > + * when returning to the interrupted context. > + */
I would first have the bit about exceptions, then mention sleeping and add something along the lines of
"[...] no sleeping functions should be called - we rely on exception handling to take care of the user_access status for us, but that doesn't happen when directly calling schedule()."
My wording's not the best but I just want something to point out *why* sleeping ain't okay.
> #ifndef user_access_begin > #define user_access_begin() do { } while (0) > #define user_access_end() do { } while (0) > @@ -273,6 +280,10 @@ static inline unsigned long __copy_from_user_inatomic_nocache(void *to, > #define unsafe_put_user(x, ptr, err) do { if (unlikely(__put_user(x, ptr))) goto err; } while (0) > #endif > > +#ifndef unsafe_user_region_active > +#define unsafe_user_region_active() false > +#endif > + > #ifdef CONFIG_HARDENED_USERCOPY > void usercopy_warn(const char *name, const char *detail, bool to_user, > unsigned long offset, unsigned long len); > diff --git a/kernel/sched/core.c b/kernel/sched/core.c > index 6fedf3a..03f53c8 100644 > --- a/kernel/sched/core.c > +++ b/kernel/sched/core.c > @@ -3289,6 +3289,13 @@ static inline void schedule_debug(struct task_struct *prev) > __schedule_bug(prev); > preempt_count_set(PREEMPT_DISABLED); > } > + > + if (unlikely(unsafe_user_region_active())) { > + printk(KERN_ERR "BUG: scheduling while user_access enabled: %s/%d/0x%08x\n", > + prev->comm, prev->pid, preempt_count()); > + dump_stack(); > + } > + > rcu_sleep_check(); > > profile_hit(SCHED_PROFILING, __builtin_return_address(0)); > @@ -6151,6 +6158,18 @@ void ___might_sleep(const char *file, int line, int preempt_offset) > EXPORT_SYMBOL(___might_sleep); > #endif > > +void __might_resched(const char *file, int line) > +{ > + if (!unsafe_user_region_active()) > + return; > + > + printk(KERN_ERR > + "BUG: rescheduling function called from user access context at %s:%d\n", > + file, line); > + dump_stack(); > +}
So this check is "careful, things might go bad" and the schedule_debug() one is "things went bad". IIUC we'll always get this warning when we hit the schedule_debug() one. I was going to suggest only keeping one of them, but I think both hold value.
> +EXPORT_SYMBOL(__might_resched); > + > #ifdef CONFIG_MAGIC_SYSRQ > void normalize_rt_tasks(void) > { > -- > 1.9.1 >
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